The need for recovery of organic compounds from aqueous solutions occurs in the manufacture of organic chemicals in fermentation processes and from biologically produced organic compounds, such as via recombinant DNA, in manufacture of organic chemicals by conventional oxidation processes, and in the handling of some aqueous effluent streams. For example, organic compounds such as alcohols, aldehydes, ketones, ethers, carboxylic acids, esters, amines and the like, are often produced in these various manufactures and typically are dissolved as solutes in dilute aqueous solution.
Recovery of acetic acid from water has long been an important fluid separation, and known processes include liquid-liquid extraction, azeotropic distillation, and extractive distillation; however, there has usually been a large energy cost per unit of acetic acid recovered. Several conventional separation processes for acetic acid are surveyed by C. Judson King, Handbook of Solvent Extraction, "Acetic Acid Extraction", pp. 567-573 (1983).
Recovery of ethanol from dilute aqueous solutions has received particular attention recently in view of the potential feasibility of using a gasoline-anhydrous ethanol blend ("gasohol"). Various processes for obtaining ethanol from dilute aqueous solution are described in U.S. Pat. No.4,450,294, inventor Feldman, issued May 22, 1984.
Adsorption processes are coming into use for recovery of dissolved organic compounds from aqueous solution, especially for applications such as fermentation processes, since solvent extraction is complicated by the possibility of contamination of the aqueous stream by residual dissolved or emulsified solvent. Contamination is avoided in adsorption processes because of the insolubility of the solid, adsorbent phase. In such adsorption processes, water is necessarily taken up along with the dissolved organic compound because of competitive adsorption onto the surfaces, pore-filling, and hold-up in interstices. For example, adsorption of acetic acid from a 4 wt.% aqueous solution with activated carbon typically gives about 15% to 30% of acetic acid (with 85% to 70% being water) in the adsorbate, on a carbon-free basis.
Conventional regeneration processes typically entail leaching the retained adsorbate away from the adsorbent with a simple solvent (e.g. liquid methanol or acetone), or vaporization of the solute in water from the adsorbent bed by applying heat, reduced pressure, and/or a non-condensible carrier gas. Subsequent separation of co-adsorbed water from the desired organic compound and the solvent is then typically done by a series of conventional distillation steps. However, this can be difficult if the boiling point of the organic compound is close to that of water, if the organic compound forms an azeotrope with water, or if the organic compound is unstable at distillation temperatures. For example, acetic acid (b.p. 118.degree. C.) and ethanol (b.p. 79.degree. C.) have boiling points close to that of water. A number of the organic compounds of interest for recovery from aqueous solutions also have one or more of these properties, and thus are difficult to separate by conventional distillation.